The use of x-ray contrast material is increasing in an exponential fashion. The development of new media has already produced encouraging evidence of substantial diminution of chemotoxic changes that might be attributed to x-ray contrast media injection. There is, however, no substantial evidence of any diminution in the anaphylactoid systemic reactions thought most likely to be due to an idiosyncratic response. Limited clinical evidence and some animal studies have suggested that systemic anaphylactoid reactions can be prevented by pretreatment with either adrenalcorticosteroids or antihistamines. However, no control studies have been carried out thus far. We propose to undertake a large multi-institutional study to provide a sufficient number of patients to test the efficacy of pre-treatment with either adrenalcorticosteroids or antihistamines in comparison with placebo pre-treatment. It is proposed to recruit approximately 200,000 patients undergoing intravenous pyelography over a two-year period who will be studied in a double-blinded fashion to determine whether pre-treatment with adrenal corticosteroids or antihistamines has any significant advantage over pre-treatment with a placebo.